Patch library, since it always compiles in bootstrapped mode

It's unclear why it wasn't failing before, but now it does.
Patch the problem locally, but this patch will be subsumed when we make
the compiler bootstrapping.

library/src/scala/tasty/util/ShowSourceCode.scala

@@ -1142,7 +1142,9 @@ class ShowSourceCode[T <: Tasty with Singleton](tasty0: T) extends Show[T](tasty
       case '"' => "\\\""
       case '\'' => "\\\'"
       case '\\' => "\\\\"
-      case _ => if (ch.isControl) "\\0" + Integer.toOctalString(ch) else String.valueOf(ch)
+      case _ =>
+        // TODO(abeln): remove `asInstanceOf` once we do the more general bootstrapping of the compiler.
+        if (ch.isControl) "\\0" + Integer.toOctalString(ch) else String.valueOf(ch).asInstanceOf[String]
     }
 
     private def escapedString(str: String): String = str flatMap escapedChar